First-principles determination of ionic conductivity in crystalline and amorphous LiNbCl6 solid-state electrolytes for lithium batteries

Solid-state electrolytes with high ionic conductivity are key to advancing solid-state lithium-ion batteries. Among ternary halides, LiNbCl6has demonstrated some of the highest ionic conductivity reported to date. Here, we use density functional theory and ab initio molecular dynamics to model both crystalline and amorphous LiNbCl6 and to relate structure to ion transport performance. For the crystalline phase, the optimized lattice constants and angles agree well with experiment; however, a small positive decomposition energy (~0.01 eV/atom) and imaginary phonon modes indicate that the crystal is metastable. We generated an amorphous LiNbCl6 structure using a melt-quench protocol, validated by the radial distribution function (RDF). The amorphous phase exhibits a calculated ionic conductivity of 14.69 mS/cm, in close agreement with the experimental value of 12.19 mS/cm, and an activation barrier of 0.21 eV, comparable to the measured 0.15 eV. In contrast, the crystalline phase shows substantially lower conductivity. Analysis of the van Hove function of Li ions and the Li-Cl RDF suggests that both the availability of connected migration sites and the fraction of mobile Li ions are primary factors underlying the conductivity enhancement in the amorphous structure. These results clarify the structural origins of fast ion transport in halide electrolytes and provide guidance for designing high-conductivity solid-state electrolytes for lithium batteries.